Fix style around header inclusion

This change is to be in sync with other libraries in system/core.

External headers are included via #include <...> rather
than #include "...", which is for headers from the current library.

Bug: 130388701
Test: build & pass presubmit tests

Change-Id: Ifb87ebaea1b0ff95e2e79352e6a42e3bfd949c52